Estimates the covariance matrix.
Normalization is by (N1) where N is the number of observations (unbiased estimate). If bias is True then normalization is by N.
By default, masked values are recognized as such. If x and y have the same shape, a common mask is allocated: if x[i,j] is masked, then y[i,j] will also be masked. Setting allow_masked to False will raise an exception if values are missing in either of the input arrays.
Parameters:  x : array_like
y : array_like, optional
rowvar : {False, True} optional
bias : {False, True} optional
allow_masked : {True, False} optional


Raises:  ValueError: :
